This invention relates to a piston having a crown, a head land depending downwardly from the crown, a skirt and at least one groove between the skirt and the head land to receive a piston ring.
Typically, a plurality of lands, separated by grooves, are provided between the crown and the skirt. For example, the piston may have three lands, a head land next to the crown, an intermediate land and a bottom land next to the skirt, there being a first compression ring in a groove between the head and intermediate lands, a second compression ring in a grrove between the intermediate and bottom lands and an oil control ring in a groove between the bottom land and the skirt. However, a different number of lands may be provided, for example, there may be only a head land between the crown and skirt with a single groove therebetween with a single ring therein.
Hitherto such pistons have been made so that, in use at normal operating temperature, there is a clearance between the head land and a cylinder bore, in which the piston is used, which is greater than the clearance between the skirt and the cylinder. When the piston is cold the difference between the clearances is greater because there is a falling thermal gradient between the crown of the piston and the bottom of the skirt so that there is a greater extent of thermal expansion of the piston towards the crown.
As a result, the head land normally runs at relatively large clearances relative to the cylinder wall, for example at 0.40-1.00 mm diametral clearance with pistons having a diameter of about 50 mm-100 mm. This relatively large clearance is to ensure that contact with the cylinder bore is avoided throughout the thermal operating range of the piston as well as to avoid contact as a result of tipping of the piston which can occur, particularly when the engine is cold.
This relatively large clearance results in the following three disadvantages.
Even at maximum operating temperatures there is a "dead gas zone" in the clearance region between the head land and the cylinder bore which can lead to detonation and result in land burning and result in local melting of the land as well as damage to the top ring. Often breakage of both the land and the ring occurs. There is an increasing trend to operate internal combustion engines with lean mixtures for fuel economy and to reduce exhaust emissions, especially in engines having two valves per cylinder and this exacerbates the problem as the spark plug is located to one side of the cylinder and land burning occurs on the opposite side at the maximum distance from the spark plug.
The relatively large head land clearance exposes the top ring to combustion gases which can lead to ring collapse and reduced tension of the spring and higher than desired blow-by of gases which leads to reduced efficiency and contamination of the lubricating oil of the engine by the products of combustion.
There is a current trend to make pistons with a relatively high diameter to height ratio so that the length of the skirt available to support the piston is relatively small thus weight and minimising the area of the piston available for frictional restraint by the bore. As a result of the relatively large clearance provided between the head land and the cylinder bore there is a relatively great tendency for such pistons to tip, especially with so-called "slipper" designs, which have not only short skirt support areas, but also skirt support areas which are narrow in the thrust-trailing axis so allowing additional movement of the piston along the gudgeon pin axis and poor control.